A U.S. court on Thursday ruled that the formulation patent for AstraZeneca's big-selling anti-psychotic drug Seroquel XR was valid and had been infringed by Mylan and other companies seeking to sell cheap generic versions of the medicine.

The judgment by the U.S. District Court for the District of New Jersey is limited to the U.S. market only, AstraZeneca said.

Britain's High Court last week found AstraZeneca's patent on extended-release Seroquel to be invalid. That ruling is only binding in Britain.

Seroquel XR, a longer acting version of Seroquel which also treats depression and bipolar disorder, has annual U.S. sales of about $750 million.

Mylan earlier this week said it had begun selling generic versions of regular Seroquel. AstraZeneca's patents on the extended release version of the drug will expire in 2017.

The latest patent ruling applies to Mylan as well as drugmakers Anchen Pharmaceuticals, Osmotica Pharmaceutical and Torrent Pharmaceuticals.

AstraZeneca last year settled its patent infringement suit against Handa Pharmaceuticals and Accord Healthcare, granting those companies a license to a key Seroquel XR patent effective in late 2016.

Britain's second-biggest drugmaker has been trying to switch patients over to the newer version ahead of the arrival of generic forms of basic Seroquel.

Worldwide annual sales of the Seroquel franchise were $5.3 billion in 2010, making the medicine the company's second-biggest seller behind cholesterol fighter Crestor.